This invention relates to methods of forming circuitry fabrication masks having a subtractive alternating phase shift region.
In semiconductor manufacturing, photolithography is typically used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light or electron beam is passed through a mask/reticle and onto the semiconductor wafer. The mask contains light restricting regions (for example totally opaque or attenuated/half-tone) and light transmissive regions (for example totally transparent) formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel-spaced conductive lines on a semiconductor wafer.
The wafer is provided with a layer of energy sensitive resist material, for example photosensitive material commonly referred to as photoresist. Ultraviolet light passed through the mask onto the layer of photoresist transfers the mask pattern therein. The photoresist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist. The remaining patterned resist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as ion implantation or etching relative to layers on the wafer beneath the resist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and in the dimensions of conductor elements which connect those integrated circuit devices. The wavelength of coherent light employed in photolithographic processes by which integrated circuit devices and conductors are formed has typically desirably been smaller than the minimum dimensions within the reticle or mask through which those integrated circuit devices and elements are printed. At some point, the dimension of the smallest feature opening within the reticle approaches the wavelength of coherent light to be employed. Unfortunately, the resolution, exposure latitude and depth of focus in using such reticles and light decreases due to aberrational effects of coherent light passing through openings of width similar to the wavelength of the coherent light. Accordingly as semiconductor technology has advanced, there has traditionally been a corresponding decrease in wavelength of light employed in printing the features of circuitry.
One approach for providing high resolution printed integrated circuit devices of dimensions similar to the wavelength of coherent light utilized employs phase shift masks or reticles. In comparison with conventional reticles, phase shift masks typically incorporate alternating thicker and thinner transparent regions within the conventional chrome metal-on-glass reticle construction. These shifter regions are designed to produce a light transmissive substrate thickness related to the wavelength of coherent light passing through the phase shift mask. Specifically, coherent light rays passing through the light transmissive substrate and the shifter regions have different optical path lengths, and thus emerge from those surfaces with different phases. By providing light transmissive shifter regions to occupy alternating light transmitting regions of the patterned metal layer of a conventional phase shift mask of the Levenson type, adjacent bright areas are formed preferably 180xc2x0 out-of-phase with one another. The interference effects of the coherent a light rays of different phase provided by a phase shift mask form a higher resolution image when projected onto a semiconductor substrate, with accordingly a greater depth of focus and greater exposure latitude.
Etching can be used to form the thinner regions of the light transmissive substrate. Such typically employs a timed dry anisotropic etch of the typical quartz substrate in one or more steps in an effort to achieve a near perfect 180xc2x0 phase shift. The dry etching can cause quartz roughness which can result in transmission and phase error. Further, the etch may not be as anisotropic a s desired, resulting in an inwardly tapering xe2x80x9cVxe2x80x9d shape, which can also cause transmission errors. Further with a timed dry etch, it can be difficult to control the phase shift achieved. Further, defects can be created in the dry etching the density of which is hard to control.
Further and regardless, the etched spaces even if perfectly anisotropic typically transmit less light than adjacent unetched spaces due to total internal reflection along the sidewalls. Accordingly, the resultant image can consist of unevenly spaced lines/devices in spite of equal spacing on the mask.
A number of different techniques have been proposed to balance the transmission of etched versus adjacent unetched spaces. Such include blanket wet etching the entire reticle to enlarge both the etched and unetched spaces to undercut the chrome layer as well as widen the base of the quartz material. Yet, isotropically wet etching a reticle can create large masking layer overhangs and must be carefully timed.
Another method includes selectively wet etching just the phase shift regions while masking the non-shifted regions. Again, careful timing control is important to avoid over etching. Another method involves dry etching both shifted and non-shifted regions. Unfortunately, this can reduce the transmission of light passing through both the shifted and non-shifted regions, thus degrading the image quality.
Another method utilized in an effort to overcome the above-described problem is known as xe2x80x9cdata biasingxe2x80x9d. Here, the spacing on the mask between features is modified from that desired in the wafer such that the resultant desired intensity is achieved in the wafer and thereby the desired circuitry pattern is created.
Example prior art alternating phase shift mask fabrication techniques and masks are described in Uwe A. Griesinger et al., Transmission and Phase Balancing of Alternating Phase Shifting Masks (5xc3x97)xe2x80x94Theoretical and Experimental Results, SPIE99 #3873-36, pp. 1-11; and in Christophe Pierrat et al., Phase-Shifting Mask Topography Effects on Lithographic Image Quality, SPIE Vol. 1927 Optical/Laser Microlithography VI (1993), pp. 28-41.
The invention includes methods of forming a circuitry fabrication mask having a subtractive alternating phase shift region. In one implementation, a light shielding layer is formed over a light transmissive substrate. The light shielding layer is patterned to have at least a portion having a first series of openings which alternate with a second series of openings. One of the first series of openings or the second series of openings is effectively masked while leaving the other of the first series of openings or the second series of openings effectively unmasked. While the one openings are effectively masked and the other openings are effectively unmasked, a species is ion implanted into the light transmissive substrate through the other openings. The implanted species effectively increases a wet etch rate of the light transmissive substrate in a wet etch chemistry compared to light transmissive substrate which is not effectively implanted with the species. The species implanted light transmissive substrate is wet etched through the other openings substantially selective to light transmissive substrate which is not effectively implanted with the species effective to at least partially form alternating phase shift regions through the other openings as compared to the alternating one openings.
Other implementations and aspects are contemplated.